An anti-lock braking system (hereafter called ABS) is one of the important parts of a vehicle. FIG. 5 illustrates a hydraulic anti-lock braking system. The anti-lock braking system comprises a number of wheels 10, a plurality of wheel sensors 20, an electrical control unit (ECU) 30, a motor pump assembly 40 and a hydraulic system 50. The hydraulic system is used to control braking of the wheels. The wheel sensors 20 detect rotation of the wheels. The ECU 30 generates a control signal based on the detection results provided by the wheel sensors 20. The control signal is used to control the motor pump assembly 40 and valve of the hydraulic system 50 to unlock the wheels when a skidding situation is determined.
The motor pump assembly 40 comprises a motor and a pump. The motor comprises a rotatable output shaft. A bearing is fixed onto one end of the output shaft. The rotational axis of the output shaft is offset from a mechanical centerline of the bearing. For example, the output shaft is a concentric shaft, and the bearing is an eccentric bearing. The pump comprises two pistons. Each of the pistons is attached to the bearing in a substantially radial direction. The two pistons are driven by the bearing when the bearing rotates with the output shaft. Permanent magnet direct current (hereafter called PMDC) motors are widely used in anti-lock braking systems. A traditional PMDC motor comprises a stator having four stator poles (hereafter called poles) and a rotor having a rotor core with thirteen rotor poles or teeth (hereafter called slots). Windings of the rotor comprise a plurality of coils. Each coil is wound about three of four teeth (coil pitch is equals to 3 or 4). The coils overlap at both axial ends of the rotor core, resulting in a motor with a large axial length.
It is desired to develop a motor with a shorter axial length but with a substantially equivalent performance.